A goblet contains $3$ red marbles, $9$ green marbles, and $6$ blue marbles. If a marble is randomly chosen, what is the probability that it is not green?
There are $3 + 9 + 6 = 18$ marbles in the goblet. There are $9$ green marbles. That means $18 - 9 = 9$ are not green. The probability is $ \frac{9}{18} = \dfrac{1}{2}$.